Such a network is already known in the art, e.g. from the paper "Architectural design of on-demand video delivery systems: the spatio-temporal storage allocation problem", by R. Ramarao et al., Proc. ICC '91, June 1991, pp. 506-511.
In the latter paper it is explained that the video on demand service cannot be efficiently provided by a single central server since the retrieval costs of the video signals are in that case prohibitively large. Indeed, in providing the service via this single central server to geographically distributed user stations clearly resources are wasted in transmitting the video signals over long distances.
Therefore a network of the above distributed nature is proposed in which an optimization of the retrieval costs can be made as a function of the local popularity of the video signals offered by the network. This known network further is hierarchical in that the intermediate layer of distributed servers is coupled to the lower layer of user stations and to an upper layer which comprises the central server. Thanks to these features the locally popular material, i.e., the second pluralities of video signals, may be stored locally whereas the full video archive, i.e., the first plurality of video signals, may be stored in a central location from which the locally popular video material of the second pluralities may then be updated dependent upon demand.
The above paper describes in particular how the video signals should preferably be distributed over the various layers but does not propose an architecture for these servers. Known architectures, for instance the one described in the paper "A store-and-forward architecture for video on demand service", by Gelman et al., ICC '91, June 1991, pp. 842-846, cannot be used for the above distributed network. Indeed, although this architecture includes a centralized information warehouse, which may be considered as a central server, connected to central offices which include service circuits dedicated to the video on demand service, the latter circuits cannot be considered as the above defined distributed servers. This is due to the presence in these service circuits of buffers which do not allow storage of a complete video signal corresponding for instance to a motion picture. Therefore the service circuits do not operate as autonomous distributed servers since for the transmission of each video signal they need to interact with the central server.
From the above it follows that the distributed servers need to be defined in more detail in order to reap the benefits of a distributed network of the above type. More particularly, the distributed servers have to be designed as a function of their location and function in the above distributed and hierarchical network and in view of both their installation and operation costs.